Method for preparing a dialkyl carbonate, and its use in the preparation of diaryl carbonates and polycarbonates

ABSTRACT

Unexpected corrosion of downstream sections of a dialkyl carbonate manufacturing apparatus has been traced to alkyl chloroformate impurities, which slowly decompose to yield hydrochloric acid. An improved process and apparatus for dialkyl carbonate synthesis reduce corrosion by physically removing or chemically decomposing the alkyl chloroformate impurities within the corrosion-resistant upstream sections of the apparatus.

BACKGROUND OF INVENTION

[0001] Polycarbonate resins are useful materials valued for theirphysical and optical properties. Methods for the preparation ofpolycarbonate resins include interfacial processes and melt processes.In interfacial processes, as described, for example, in U.S. Pat. No.4,360,659 to Sikdar, a bisphenol is reacted with phosgene in thepresence of solvents. In melt processes, as described, for example, inU.S. Pat. No. 3,153,008 to Fox, a bisphenol is reacted with a diarycarbonate. Melt processes are presently preferred because they avoid theuse of phosgene and solvents.

[0002] Use of a melt process for polycarbonate synthesis requires anindustrially efficient process for producing diaryl carbonates. Thereare several known processes for producing diaryl carbonates. One exampleof such a process is described by U.S. Pat. No. 4,182,726 to Illuminatiet al. In this process, diaryl carbonates are produced by reactingdialkyl carbonates with aryl hydroxides (see Scheme I, below).

[0003] U.S. Pat. No. 4,182,726 also demonstrates that diary carbonatescan be reacted together with dihydric phenols to produce polycarbonates(see Scheme II, below).

[0004] A preferred process for making dialkyl carbonates is illustratedin Scheme III, below, and described, for example, in U.S. Pat. No.5,527,943 to Rivetti et al.; and U.S. Pat. Nos. 4,218,391 and 4,318,862to Romano et al.

[0005] U.S. Pat. No. 5,527,943 (the '943 patent) also describes a knowndrawback of the dialkyl carbonate process according to Scheme (III): itproduces water as a by-product. Also, hydrochloric acid (HCl) may becontinuously added to the reaction mixture to maintain a desired molarratio of chloride to copper. Therefore, HCl, CuCl catalyst, and waterare typically found in the stream exiting the reactor vessel.Hydrochloric acid and copper chlorides are very corrosive in thepresence of water, so equipment made from corrosion-resistant materials,such as glass-lined vessels, must be used in the reaction section of achemical plant making dialkyl carbonates by this process. Ascorrosion-resistant equipment is expensive, there is a desire to use itin as little of the plant as possible.

[0006] A typical plant for performing preparing dialkyl carbonatesaccording to Scheme Ill may contain three sections: a reaction sectionfor converting raw materials to dialkyl carbonate, a separation sectionfor isolating the dialkyl carbonate from unreacted monomers andby-products, and a purification section for removing water and furtherisolating the dialkyl carbonate. The '943 patent teaches that one canminimize the amount of corrosion-resistant equipment required byremoving the HCl from the process stream immediately after the reactionsection. This eliminates the necessity of using expensivecorrosion-resistant materials in the separation and purificationsections of the plant. The '943 patent further suggests that removal ofHCl and possible copper halide salts from the stream immediately afterthe reaction section can be accomplished by exposing the gas-liquidmixture produced by the reaction to a liquid stream consisting of one ofthe process fluids. The '943 patent also states that the operatingconditions employed are preferably adjusted such that the gaseousmixture from the reactor does not condense, or condenses only to anegligible extent, before the acid removal section in order to avoid thenecessity of having to reheat the mixture before removing the HCl (col.3, lines 17-30).

[0007] In view of the above, it was desirable to construct a plantwherein the HCl and any copper halide salts would be removed from thestream after the reaction section to avoid corrosion in the separationand purification sections. However, a technique similar to thatdescribed by the '943 patent—removing HCl and copper salts by treatmentof a vaporized feed in a column using a counterflowing azeotrope fluidfrom the reaction mixture—failed to prevent corrosion in the downstreamseparation and purification sections.

[0008] There is therefore a need for a dialkyl carbonate process thatrecognizes and eliminates additional sources of corrosion.

SUMMARY OF INVENTION

[0009] The above-described and other drawbacks and disadvantages of theprior art are alleviated by a method of preparing a dialkyl carbonate,comprising: reacting an alkanol, oxygen, carbon monoxide, and a catalystto form a mixture comprising a dialkyl carbonate, an alkylchloroformate, hydrochloric acid, water, carbon dioxide, and carbonmonoxide; and removing alkyl chloroformate from said mixture.

[0010] After considerable effort, the present inventors have discoveredthat dialkyl carbonate synthesis can form alkyl chloroformateby-products that lead to problematic corrosion. For example, in thereaction of methanol, carbon monoxide, and oxygen to form dimethylcarbonate (hereinafter “DMC”), methyl chloroformate (hereinafter “MCF”)may be formed as a by-product. The MCF may pass through the HCl removalcolumn into the separator and purification sections, where it reactsslowly with methanol and/or water to form corrosive HCl. Therefore, itwas determined that steps were needed to remove MCF prior to theseparation and purification sections.

[0011] Other embodiments, including an apparatus for preparing dialkylcarbonates, are described below.

BRIEF DESCRIPTION OF DRAWINGS

[0012]FIG. 1 is a diagrammatic view of a first embodiment of theapparatus.

[0013]FIG. 2 is a simplified diagrammatic view of a comparativeapparatus that is susceptible to corrosion.

[0014]FIG. 3 is a simplified diagrammatic view of an embodiment of theapparatus in which the fluid passageway 110 comprises two holdingvessels 120.

[0015]FIG. 4 is a simplified diagrammatic view of an embodiment of theapparatus in which the fluid passageway 110 comprises four holdingvessels 120.

[0016]FIG. 5 is a simplified diagrammatic view of an embodiment of theapparatus in which the fluid passageway 110 comprises a tubular section130.

[0017]FIG. 6 is a simplified diagrammatic view of an embodiment of theapparatus comprising an ion exchange resin bed 190.

[0018]FIG. 7 is a simplified diagrammatic view of an embodiment of theapparatus in which the fluid passageway 110 comprises a first gas-liquidseparator 90 and a second gas-liquid separator 100.

[0019]FIG. 8 is a simplified diagrammatic view of an embodiment of theapparatus in which the fluid passageway 110 precedes the firstgas-liquid separator 90.

[0020]FIG. 9 is a simplified diagrammatic view of an embodiment of theapparatus in which the fluid passageway 110 follows the azeotrope column180.

[0021]FIG. 10 is a plot of chloride concentrations at the bottom of anazeotrope column 180 as a function of apparatus type (FIG. 2 and FIG. 3)and time.

[0022]FIG. 11 is a plot of methyl chloroformate concentrations enteringand exiting the fluid passageway 110 as a function of time for anapparatus corresponding to FIG. 3.

DETAILED DESCRIPTION

[0023] One embodiment is a method, comprising: reacting an alkanol,oxygen, carbon monoxide, and a catalyst to form a mixture comprising adialkyl carbonate, an alkyl chloroformate, hydrochloric acid, water,carbon dioxide, and carbon monoxide; and removing alkyl chloroformatefrom said mixture.

[0024] There is no particular limitation on the alkanol used in themethod. Suitable alkanols include primary, secondary, and tertiaryC₁-C₁₂ alkanols, with primary C₁-C₆alkanols being preferred. Highlypreferred alkanols include methanol.

[0025] Oxygen may be provided in any form, with gaseous forms beingpreferred. Suitable oxygen sources include, for example, air, andoxygen-containing gases having at least about 95 weight percentmolecular oxygen, preferably at least about 99 weight percent molecularoxygen. Suitable oxygen-containing gases are commercially availablefrom, for example, Air Products.

[0026] Carbon monoxide is preferably supplied as a gas having at leastabout 90 weight percent, preferably at least about 95 weight percent,more preferably at least about 99 weight percent, carbon monoxide.Suitable carbon monoxide-containing gases are commercially availablefrom, for example, Air Products.

[0027] Suitable catalyst include those comprising iron, copper, nickel,cobalt, zinc, ruthenium, rhodium, palladium, silver, cadmium, rhenium,osmium, iridium, platinum, gold, mercury, and the like, and combinationscomprising at least one of the foregoing metals. Preferred catalysts maycomprise copper. A highly preferred catalyst comprises copper andchloride ion in a molar ratio of about 0.5 to about 1.5. Within thisrange, a molar ratio of at least about 0.8 may be preferred. Also withinthis range, a molar ratio of up to about 1.2 may be preferred. Highlypreferred catalysts include cuprous chloride (CuCl) and cupric chloride(CuCl₂), with cuprous chloride being more highly preferred. Duringoperation of the process, a suitable chloride ion concentration may bemaintained by the addition of hydrochloric acid (HCl).

[0028]FIG. 1 illustrates a dialkyl carbonate plant 10 having linkedreaction section 20, separation section 30, and purification section 40.With reference to FIG. 1, the catalyzed reaction of alkanol, oxygen, andcarbon monoxide may be performed in a single reactor 50, or in two ormore reactors 50. The conditions for performing this step should beselected to maximize the yield of dialkyl carbonate while minimizing thedegradation of dialkyl carbonate. Preferably, the reaction is performedin a single reactor 50, at a temperature of about 50° C. to about 250°C. Within this range, the temperature may preferably be at least about100° C. Also within this range, the temperature may preferably be up toabout 150° C. The reactor 50 is preferably kept at a pressure of about15 to about 35 bar gauge (barg). Within this range, a pressure of atleast about 20 barg may be preferred. Also within this range, a pressureup to about 28 barg may be preferred. In the case of dual reactorsystems, the catalyst may be recycled between tanks. The catalystconcentration should be sufficiently high to produce an acceptableyield, but should be kept below a concentration that would cause solidsetting of the catalyst in the reactor 50 or clogging of the equipment.The reactants alkanol, oxygen, and carbon monoxide are preferably addedin a molar ratio of (about 0.5 to about 0.7):(about 0.04 to about0.06):(about 0.8 to about 1.2), respectively. A highly preferred molarratio of alkanol:oxygen:carbon monoxide is (about 0.6):(about0.05):(about 1).

[0029] The amount of catalyst used relative to the reactants will dependon the identity of the catalyst. For example, when the catalystcomprises CuCl, a highly preferred catalyst concentration is about 140to about 180 grams per liter of reaction mixture. During operation, thecatalyst may initially be added from a catalyst tank 60. Sufficient HClis preferably added to reactor 50 from a hydrochloric acid tank 70during the course of the reaction to maintain a molar ratio of Cu:Clclose to 1.0. The concentration of HCl is preferably continuouslydetermined and controlled by the addition of HCl. A typical mass ratiofor HCl feed to total liquid feed is about 6×10⁻⁴ to about 8×10⁻⁴.

[0030] The reaction produces a mixture comprising a dialkyl carbonate,an alkyl chloroformate, hydrochloric acid, water, carbon dioxide, andcarbon monoxide. The mixture may further comprise residual methanol andoxygen, as well as side-products such as alkyl chlorides and dialkylethers. The mixture is typically withdrawn from the reactor 50 in agas/vapor form. The term “vapor” is meant to refer to gaseous organiccomponents of the mixture such as, for example, evaporated dialkylcarbonates, alcohols, alkyl chloroformates, etc., and to water vapor.That is, the term “vapor” refers to fluids having a boiling point of atleast −50° C. at one atmosphere. In contrast, the term “gas” is meant torefer to the gaseous oxygen, carbon dioxide, carbon monoxide, andoptional nitrogen. That is, the term “gas” refers to fluids having aboiling point less than −50° C. at one atmosphere. The vapor may be atleast partially condensed in condenser 80, and fed to a first gas-liquidseparator 90. The apparatus may optionally employ a single gas-liquidseparator, or a plurality of (i.e., at least 2; preferably up to about5) gas-liquid separators. The first gas-liquid separator 90 may be keptat a pressure within about 10%, more preferably within about 1%, of thepressure of the reactor 50. The gas effluent from the first gas-liquidseparator 90 may be recycled, for example to reuse excess carbonmonoxide. The mixture may be sent to a second gas-liquid separator 100,which preferably has a pressure less than about 20% of the pressure ofthe reactor 50 (e.g., preferably less than 3 bar gauge, more preferablyabout 0.2 bar gauge) to preferably achieve separation of at least about90%, more preferably at least 95%, by weight of the remaining gas in themixture. In a highly preferred embodiment, substantially all of the gasis removed from the mixture. The gas effluent removed from the secondgas-liquid separator 100 can also be recycled. It is preferred that thevapor in the mixture be in a partially condensed form (i.e., at leastabout 10% condensed), more preferably a fully condensed form (i.e., atleast about 90% condensed), before entering the first gas-liquidseparator 90, and between the first gas-liquid separator 90 and thesecond gas-liquid separator 100.

[0031] In the embodiment shown in FIG. 1, the mixture exiting the secondgas-liquid separator 100 may be in a single liquid phase. After thesecond gas-liquid separator 100, the mixture may proceed through a fluidpassageway 110 that removes alkyl chloroformate from the mixture. Itwill be understood that the terms “remove” and “removal” in reference toa particular chemical species encompass any chemical or physical processthat reduces the concentration of the species in the mixture. The alkylchloroformate may be removed from the condensate by any method. Somepreferred methods include heating, increasing pressure, increasingresidence time, adding a polar solvent, adsorbing, separating with amembrane (including gas and liquid membrane separation), pervaporating,passing through an ion exchange resin, exposing to a stoichiometricreagent, exposing to a catalytic reagent, and the like, and combinationscomprising at least one of the foregoing techniques. In a preferredembodiment, the alkyl chloroformate is removed from mixture by reactionwith water (see Scheme IV) or alkanol (see Scheme V).

[0032] It may also be preferred to remove the alkyl chloroformatewithout passing the mixture through an ion exchange resin, because suchresins are expensive to install and operate. It may be preferred toremove at least about 50 percent, more preferably at least about 90percent, yet more preferably at least about 95 percent, even morepreferably at least about 99 percent, of the alkyl chloroformate fromthe mixture. In one embodiment, it may be preferred to reduce the alkylchloroformate concentration in the mixture to less than about 500 ppm,more preferably less than about 100 ppm, yet more preferably less thanabout 30 ppm. In any of these embodiments, it may be preferred to removeless than about 10%, more preferably less than about 5%, yet morepreferably less than about 1%, of the dialkyl carbonate. Although themethod may be described as “removing less than about 10% of said dialkylcarbonate”, it will be understood that the concentration of dialkylcarbonate need not be reduced and may even increase. For example, theconcentration of dialkyl carbonate may increase if the Scheme V reactionof alkyl chloroformate with methanol forms dialkyl carbonate faster thandialkyl carbonate decomposes due to other reactions.

[0033] Through extensive kinetic studies of the dimethyl carbonateprocess utilizing variations in factors including temperature, residencetime, water concentration, methanol concentration, and hydrochloric acidconcentration, the present inventors have found that the rate of methylchloroformate decomposition may be given by the equation (1)

−r _(MCF)=(k ₁[H₂O]+k ₂[MeOH])[MCF]  (1)

[0034] where r_(MCF) is the rate of change of the moles of methylchloroformate (MCF) per unit volume, [H₂O], [MeOH], and [MCF] are theinstantaneous concentrations of water, methanol, and methylchloroformate, respectively, in moles per unit volume, and k₁ and k₂arerate constants that vary with temperature according to equations (2) and(3), respectively

k ₁ =k ₁ ⁰ e ^(−6381/T)  (2)

k ₂=k₂ ⁰ e ⁻⁷⁶⁷³/T  (3)

[0035] where k₁ ⁰=2.09×10⁹ mL/mol-min, k₂ ⁰=4.14×10¹⁰ mL/mol-min, and Tis the temperature in degrees kelvin.

[0036] In many cases, it is valid to assume that the concentrations ofwater and methanol, and the density of the solution are essentiallyconstant. Within these general kinetic constraints, different kineticexpressions may be used for different process and apparatus types. Withknowledge of the relevant chemical reactions and rate constants providedin this application, these expressions may be readily derived by thoseof ordinary skill in the art. For example, in a batch process, the rateof methyl chloroformate decomposition may be expressed as a function ofresidence time, as shown in equation (4):

−d[MCF]/dt=(k ₁[H₂O]+k ₂[MeOH])[MCF]  (4)

[0037] where t is residence time in minutes. The residence time t may bedefined as the total time spent by an average molecule in the fluidpassageway 110. In a batch process, at least about 50% of the methylchloroformate may be removed by maintaining the mixture under conditionscomprising a water concentration ([H₂O]), a methanol concentration([MeOH]), a temperature (T), and a residence time (t), such that aparameter X according to the equation (5)

X=exp{−[(2.09×10⁹)e ^((−6381/T))[H₂O]+(4.14×10¹⁰)e^((−7673/T))[MeOH]]t}  (5)

[0038] has a value less than about 0.9, wherein the water concentrationand the methanol concentration are expressed in moles per milliliter,the temperature is expressed in degrees Kelvin, and the residence timeis expressed in minutes. The value of X may preferably be less thanabout 0.5, more preferably less than about 0.2, yet more preferably beless than about 0.1, even more preferably less than about 0.05, stillmore preferably less than about 0.01. The water concentration may beabout 0.1 to about 50 moles per liter (mol/L). Within this range, thewater concentration may preferably be at least about 0.5 mol/L, morepreferably at least about 1 mol/L. Also within this range, the waterconcentration may preferably be up to about 30 mol/L, more preferably upto about 20 mol/L, yet more preferably up to about 10 mol/L, even morepreferably up to about 5 mol/L. The methanol concentration may be about1 to about 25 mol/L. Within this range, the methanol concentration maypreferably be at least about 5 mol/L, more preferably at least about 10mol/L. Also within this range, the methanol concentration may preferablybe up to about 20 mol/L, more preferably up to about 18 mol/L. Theresidence time may be about 0.5 hour to about 10 hours. Within thisrange, the residence time may preferably be at least about 1 hours, morepreferably at least about 2 hours. Also within this range, the residencetime may preferably be up to about 8 hours, more preferably up to about6 hours. The temperature may be about 30 to about 130° C. Within thisrange, the temperature may preferably be at least about 40° C., morepreferably at least about 50° C., yet more preferably at least about 60°C. Also within this range, the temperature may preferably be up to about110° C., more preferably up to about 100° C., yet more preferably up toabout 90° C.

[0039] In the limit of an ideal steady state plug flow reactor, andassuming constant density of the mixture, the rate of methylchloroformate decomposition may be expressed according to equation (3),with t representing residence time in minutes.

[0040] For an ideal steady state continuous stirred tank reactor (CSTR),the concentration of methyl chloroformate is given by equation (6)

[MCF]=[MCF]_(t=0)(1/(1+kt))  (6)

[0041] where [MCF]_(t=0) is the initial concentration of methylchloroformate in moles per milliliter, t is the residence time inminutes, and k is given by equation (7)

k=k ₁[H₂O]+k ₂[MeOH]  (7)

[0042] where k₁, k₂, [H₂O], and [MeOH] are as defined above.

[0043] In another embodiment that relates to a batch reactor, removingalkyl chloroformate from the mixture comprises maintaining the mixtureunder conditions comprising an initial concentration of methylchloroformate ([MCF]_(t=0)), a water concentration ([H₂O]), a methanolconcentration ([MeOH]), a temperature (T), and a residence time (t),such that a parameter Z calculated according to the equation (8)

Z=[MCF] _(t=0) exp{−[(2.09×10⁹)e ^((−6381/T))[H₂O]+(4.14×10¹⁰)e^((−7673/T))[MeOH]]t}  (8)

[0044] has a value less than about 5×10⁻⁶, preferably less than about1×10⁻⁶, more preferably less than about 5×10⁻⁷, even more preferablyless than about 5×10⁻⁸, wherein the initial concentration of methylchloroformate, the water concentration, and the methanol concentrationare expressed in moles per milliliter, the temperature is expressed indegrees Kelvin, and the residence time is expressed in minutes. Thetemperature, residence time, methanol concentration, and waterconcentration in this expression are as described above. The initialconcentration of methyl chloroformate will depend on the reactorconditions, but it is typically about 5×10⁻³ moles per liter to about5×10⁻¹ moles per liter. Within this range, the initial concentration ofmethyl chloroformate may be at least about 1×10⁻² moles per liter. Alsowithin this range, the initial concentration of methyl chloroformate maybe up to about 1×10⁻¹ moles per liter.

[0045] In a preferred embodiment that relates to a batch reactor,removing alkyl chloroformate comprises subjecting the mixture toconditions comprising an initial dimethyl carbonate concentration([DMC]_(t=0)), an initial water concentration ([H2O]_(t=0)), an initialmethanol concentration ([MeOH]_(t=0)), an initial hydrochloric acidconcentration ([HCl]_(t=0)), a temperature (T), and a residence time(t), such that a parameter X calculated according to the equation (9)

X=exp{−[(2.09×10⁹)e ⁽⁻6381/T)[H₂O]_(t=0)+(4.14×10¹⁰)e^((−7673/T))[MeOH]_(t=0) ]t}  (9)

[0046] has a value less than about 0.9, and a parameter Y calculatedaccording to the equation (10) $\begin{matrix}{Y = \frac{( {1 - \frac{\lbrack {H_{2}O} \rbrack_{t = 0}}{\lbrack{DMC}\rbrack_{t = 0}}} )}{\begin{matrix}( {1 - {( \frac{\lbrack {H_{2}O} \rbrack_{t = 0}}{\lbrack{DMC}\rbrack_{t = 0}} )( {\exp( ( {6.6 \times 10^{10}} ) } }}  \\   {{{( {\exp ( {{- 6636}/T} )} )\lbrack{HCl}\rbrack}_{t = 0}\lbrack{DMC}\rbrack}_{t = 0}( {\frac{\lbrack {H_{2}O} \rbrack_{t = 0}}{\lbrack{DMC}\rbrack_{t = 0}} - 1} )t} ) ) )\end{matrix}}} & (10)\end{matrix}$

[0047] has a value of at least about 0.9, wherein the initial dimethylcarbonate concentration, the initial water concentration, the initialmethanol concentration, and the initial hydrochloric acid concentrationare expressed in moles per milliliter, the temperature is expressed indegrees Kelvin, and the residence time is expressed in minutes. Thevalue of Y may preferably be at least about 0.95, more preferably atleast about 0.99. Suitable analytical techniques to determine initialconcentrations of water, methanol, hydrochloric acid, and dimethylcarbonate in reaction mixtures are well known in the art. The term“initial concentration” refers to the concentration of a species beforeintentional removal of alkyl chloroformate. The initial water andmethanol concentrations are the same as the water and methanolconcentrations described above (under typical reaction conditions, thewater and methanol concentrations are large are essentially constantduring alkyl chloroformate removal). The initial dimethyl carbonateconcentration may be about 0.5 to about 10 mol/L. Within this range, theinitial dimethyl carbonate concentration may preferably be at leastabout 1 mol/L, more preferably at least about 2 mol/L. Also within thisrange, the initial dimethyl carbonate concentration may preferably be upto about 8 mol/L, more preferably up to about 6 mol/L. The concentrationof HCl in the mixture will depend on the type and concentration ofcatalyst employed. The initial hydrochloric acid concentration willdepend on the type and amount of catalyst, but it is typically about1×10⁻³ to about 2×10⁻¹ moles per liter. Within this range, the initialhydrochloric acid concentration may preferably be at least about 5×10⁻¹more preferably at least about 1×10⁻² mol/L. Also within this range, theinitial hydrochloric acid concentration may preferably be up to about1×10⁻¹ more preferably up to about 7×10⁻² mol/L.

[0048] The method may be operated, for example, in a batch, semi-batch,or continuous manner.

[0049] In the particular embodiment shown in FIG. 1, the mixture passesthrough a first heat exchanger 140 to adjust the temperature of themixture about 30° C. to about 130° C. Within this range, the temperaturemay preferably be at least about 40° C., more preferably at least about50° C. Also within this range, the temperature may preferably be up toabout 80° C., more preferably up to about 70° C. The term “heatexchanger” describes a well-known device for heating chemical reactionstreams, typically by exchanging heat between a thermal energy source(e.g., steam) and a cooler chemical reaction stream, but it isunderstood that other types of equivalent heaters (e.g., electricalheaters) are also included. The condensate may proceed into a fluidpassageway 110, which serves to increase the residence time of themixture under conditions to maximize decomposition of alkylchloroformate while minimizing decomposition of dialkyl carbonate. Thecondensate may preferably remain fully condensed within the fluidpassageway 110. It is desirable to keep the condensate fully condensedbecause at least some alkyl chloroformates (e.g., methyl chloroformate)are more stable in the vapor phase than the liquid phase underconditions used for this process.

[0050] The residence time and temperature in the fluid passageway 110are preferably sufficient to remove enough alkyl chloroformate toprevent unacceptable downstream corrosion, but they should not be soexcessive as to cause unnecessary reductions in the productivity andyield of the desired dialkyl carbonate product. FIG. 2 shows asimplified process diagram representative of a comparison process. Inthis process, the mixture flows directly from a first gas-liquidseparator 90 to a first heat exchanger 140, then to an acid removalcolumn 160. Three specific embodiments of the fluid passageway 110 areshown in FIGS. 3, 4, and 5. In a preferred embodiment, at least about50% of the alkyl chloroformate is removed, more preferably at least 80%is removed. In a highly preferred embodiment, the alkyl chloroformateconcentration is reduced to less than about 500 parts per million (ppm)by weight, more preferably less than about 100 ppm by weight, yet morepreferably less than about 30 ppm by weight, based on the total weightof the mixture after alkyl chloroformate removal. The fluid passageway110 is preferably selected such that the total residence time betweenthe reactor 50 and the acid removal column 160 is about 0.5 hour toabout 10 hours. Within this range, the residence time may preferably beat least about 1 hour, more preferably at least about 2 hours. Alsowithin this range, the residence time may preferably be up to about 8hours, more preferably up to about 7 hours.

[0051] In one embodiment, illustrated in FIG. 3, the fluid passageway110 comprises 2 holding vessels 120. These holding vessels 120 may, forexample, maintain the mixture at a temperature of about 55° C. for about2 hours. Each holding vessel 120 may preferably have a length to volumeratio (L/V) less than 5, preferably less than about 2. While two holdingvessels 120 are illustrated in this figure, there is no particularlimitation on the number of holding vessels 120 in the fluid passageway110. It may be preferred to use at least 2 holding vessels 120, andconfigurations comprising 3, 4, 5, 6, or more holding vessels 120 mayalso be preferred.

[0052] In another embodiment, illustrated in FIG. 4, the fluidpassageway 110 comprises 4 holding vessels 120. These holding vessels120 may, for example, maintain the mixture at a temperature of about 70°C. for about 4 hours. Each holding vessel 120 may preferably have alength to volume ratio (L/V) less than 5, preferably less than about 2.

[0053] In yet another embodiment, illustrated in FIG. 5, the fluidpassageway 110 may comprise a section having L/V of at least 5,preferably at least about 10. For brevity, this section may be referredto as a tubular section 130. Such a tubular section 130 having L/V>5 maypromote plug flow of the mixture through the fluid passageway 110,thereby efficiently utilizing the residence time for removal of thealkyl chloroformate. In this embodiment, it may be preferred that themixture reside in one or more narrow sections having L/V>5 for at leastabout 50% of the total residence time spent in the fluid passageway 110,more preferably at least about 80% of the total residence time spent inthe fluid passageway 110.

[0054] Referring again to FIG. 1, after exiting the fluid passageway110, the mixture may, optionally, pass through a second heat exchanger150 to at least partially vaporize the mixture. This second heatexchanger 150 may have a residence time of less than 10 minutes. Thisvaporization step may also be accomplished without a heat exchanger bylowering the pressure applied to the condensed mixture (e.g., by passingthe condensate into an acid removal column 160 that is kept at arelatively lower pressure). The vaporized mixture may then, optionally,be treated to remove HCl, preferably by injecting it into an acidremoval column 160. The acid removal column 160 may also help remove anyentrained catalyst (e.g., CuCl) that could otherwise contribute todownstream corrosion. In the acid removal column 160, the vaporizedcondensate may preferably encounter a counter-flowing liquid supplied bycounter-flowing liquid line 170 to a higher point in the column (e.g.,the upper third). The counter-flowing liquid may trap the remaining HCland other reactants, which may be removed from the bottom of the acidremoval column 160 and recycled to the reactor 50. The dialkyl carbonatemixture may be removed from the top of the acid column 160, and,optionally, passed into an azeotrope column 180. As shown in FIG. 6, anoptional ion exchange resin bed 190 may be included after the acidremoval column 160, or at any other position downstream with respect tothe acid removal column 160. It may be advantageous to include anoptional ion exchange resin bed 190 after water is removed from theproduct dialkyl carbonate stream in the purification section 40. In apreferred embodiment, the apparatus does not include an ion exchangeresin bed 190.

[0055] In a preferred, embodiment, the method comprises reducing theconcentration of hydrochloric acid in the mixture to less than about1×10⁻³ mol/L, more preferably less than about 5×10⁻⁴ mol/L, even morepreferably less than about 1×10⁻⁴ mol/L, based on the total compositionafter removing hydrochloric acid.

[0056] In a preferred embodiment, the portions of the separation section30 downstream from the azeotrope column 180, and the purificationsubsection 40 are not required to be corrosion-resistant. Equipmentupstream of the azeotrope column 180 is preferably corrosion-resistant;for example, it may be glass lined. The term “corrosion-resistant” ismeant to describe a material capable of withstanding an HCl content of500 ppm at a temperature of about 50° C. to about 135° C. in thereaction mixture without substantial corrosion in a relatively brieftime period (e.g., six months). Glass lined vessels, precious metal(e.g., tantalum) lined vessels and special steels such as HASTELLOY® andCHROMALLOY® would be considered corrosion-resistant materials, whileordinary stainless steels not modified to enhance corrosion resistancewould not be considered corrosion-resistant. The azeotrope column 180can be made at least in part from corrosion-resistant metals. In apreferred embodiment, the bottom of the azeotrope column 180 may be madefrom a corrosion-resistant steel, whereas the top of the column can beordinary stainless steel.

[0057] In one embodiment of the apparatus, illustrated in FIGS. 1 and3-6, alkyl chloroformate is removed in a fluid passageway 110.

[0058] In another embodiment of the apparatus, illustrated in FIG. 7,the mixture is present in the gas-liquid separation vessels 90 and 100for sufficient residence time and at sufficient temperature to removealkyl chloroformate. In other words, the fluid passageway 110 comprisesthe gas-liquid separation vessels 90 and 100. For example, the mixturemay remain in the condense phase in the gas-liquid separation vessels tobe substantially decomposed by reactions with water and methanol. Inthis embodiment, the first heat exchanger 140 and the holding vessels120 may be unnecessary.

[0059] In another embodiment of the apparatus, illustrated in FIG. 8,the alkyl chloroformate may be removed in a fluid passageway 110 thatprecedes the gas-liquid separation vessels 90 and 100. In thisembodiment, one of the above-mentioned techniques for removing alkylchloroformate may be employed upstream of the gas-liquid separationvessels 90 and 100.

[0060] In another embodiment of the apparatus, illustrated in FIG. 9,the hydrochloric acid may be removed from the mixture before removingthe alkyl chloroformate. In this embodiment, the alkyl chloroformate maybe removed in the vapor, rather than the liquid phase. For example,referring to FIG. 9, the fluid passageway 110 may follow the azeotropecolumn 180; for example, it may be inserted into the azeotrope columnvapor exit line 210. In this embodiment, the first heat exchanger 140and the holding vessels 120 illustrated in FIG. 3 may be omitted. Inthis embodiment, the fluid passageway 110 may preferably comprise anapparatus suitable for removing alkyl chloroformate from the vapor phase(e.g., ion exchange resins, absorption beds, vapor phase membranes,etc.), and the alkyl chloroformate need not be condensed.

[0061] A preferred embodiment is a method of preparing a dialkylcarbonate, comprising: reacting an alkanol, oxygen, carbon monoxide, anda catalyst to form a mixture comprising a dialkyl carbonate, an alkylchloroformate, hydrochloric acid, water, carbon dioxide, and carbonmonoxide; and passing said mixture through a fluid passageway 110 at atemperature of about 50° C. to about 80° C. and for a residence time ofabout 1 hour to about 10 hours.

[0062] Another preferred embodiment is an apparatus for preparing adialkyl carbonate, comprising: means for reacting an alkanol, oxygen,carbon monoxide, and a catalyst to form a mixture comprising a dialkylcarbonate, an alkyl chloroformate, hydrochloric acid, water, carbondioxide, and carbon monoxide; and means for removing alkyl chloroformatefrom said mixture.

[0063] Another preferred embodiment is an apparatus for preparing adialkyl carbonate, comprising: a reactor for reacting an alkanol,oxygen, carbon monoxide, and a catalyst to a produce a mixturecomprising a dialkyl carbonate, an alkyl chloroformate, hydrochloricacid, water, and carbon dioxide; and a fluid passageway 110 for removingalkyl chloroformate.

[0064] Dialkyl carbonates prepared according to the method are usefulfor the preparation of diaryl carbonates. For example, diaryl carbonatesmay be generated by the reaction of a dialkyl carbonate with an arylhydroxide (see Scheme I, above). The diaryl carbonate may in turn bereacted with a dihydric phenol to form a polycarbonate (see Scheme II,above). For example, dimethyl carbonate prepared according to the methodmay be reacted with phenoxide to form diphenyl carbonate, which in turnmay be reacted with bisphenol A to form a polycarbonate.

[0065] The invention is further illustrated by the followingnon-limiting examples.

EXAMPLE 1

[0066] A plant according to simplified FIG. 2 was built and operated toproduce dimethyl carbonate. Corrosion damage was observed in anddownstream of the azeotropic column 180. After extensiveexperimentation, it was determined that the corrosion damage was causedby methyl chloroformate passing through the acid separation column.Specifically, methyl chloroformate was found to be present in theazeotrope column 180 at a concentration of 300 parts per million (ppm)by weight.

EXAMPLES 2-5

[0067] The decomposition kinetics of methyl chloroformate were studiedunder four different conditions. A procedure for determining methylchloroformate in a sample was as follows. For Example 2, 32 milliliters(mL) of dimethyl carbonate, 10 mL of dimethyl carbonate containing 50 mgof a biphenyl internal standard 63 mL of methanol, and 5 ml of waterwere added to a 250 mL flask equipped with a thermometer, a condenser,and a port for sampling. (Toluene may be used instead of themethanol/water solution.) The resultant homogeneous solution was placedin an oil bath and the temperature of the solution was held constant at50° C. At time zero, 81.7 microliters of pure methyl chloroformate wereadded to the solution (1,000 ppm on a weight basis). Samples werewithdrawn at various time intervals and were quenched by reacting themethyl chloroformate in the sample with diisobutyl amine to convert themethyl chloroformate to N,N′-diisobutyl methyl carbamate. The amount ofN,N′-diisobutyl methyl carbamate was then analyzed via titration with astandard silver nitrate solution to quantify the amount of ionicchloride present. The amount of methyl chloroformate could then beinferred by analyzing the original sample for ionic chloride. Thedifference in chloride concentration is equal to the methylchloroformate concentration because each equivalent of methylchloroformate liberates one equivalent of ionic chloride uponderivatization. Alternatively, gas chromatography can be used for directanalysis of the N,N′-diisobutyl methyl carbamate using an internalstandard.

[0068] Table I below show the observed decomposition rate constants (k)at 50° C. for various conditions. Example 2 corresponds to the casedescribed above. Example 3 has added hydrochloric acid that is generallypresent in the authentic reaction mixture. In Example 4, the effect of asmall amount of sodium bicarbonate was tested. In Example 5, the ratioof dimethyl carbonate to methanol was held constant, but the amount ofwater was increased from 5% to 10%. The results are summarized below inTable I. TABLE I DMC MeOH (wt %) (wt %) H₂0 (wt %) Temp (° C.) k (min⁻¹)Ex. 2 45 50 5 50 0.043 Ex. 3* 45 50 5 50 0.043 Ex. 4** 45 50 5 50 0.480Ex. 5*** 43 47 10 50 0.055

[0069] Plots of the logarithm of methyl chloroformate concentrationversus time were linear, fitting a pseudo-first-order kinetic model.This behavior was observed even in the presence of hydrochloric acid,and therefore this method can be used to determine the concentration ofmethyl chloroformate in a particular sample. Comparison of Examples 2and 5 indicates that only minor variations in the rate coefficient, k,are observed when analyzing samples having water contents varying by afactor of two. Comparison of Examples 2 and 3 shows, surprisingly, thatadded HCl did not affect the observed rate of methyl chloroformatedecomposition. Comparison of Examples 2 and 4 shows that even a smallamount of base increased the reaction rate by more than ten-fold. As apractical matter, however, it may be desirable to avoid strongly basicconditions because they also may increase the decomposition rate ofdimethyl carbonate.

EXAMPLE 6, COMPARATIVE EXAMPLE 1

[0070] These experiments show that the fluid passageway 110 is effectiveto reduce the concentration of methyl chloroformate that can react toform HCl in downstream sections of the plant. With reference to FIG. 1,two samples were obtained by sampling the process fluid at differentpoints in a dimethyl carbonate plant having a configuration with a firstheat exchanger 140 and two holding vessels 120 (i.e., a configurationcorresponding to FIG. 3). The first sample (Comparative Example 1) wastaken immediately before the first heat exchanger 140. The second sample(Example 6) was taken after the second holding vessel 120 (i.e., afterthe fluid passageway 110). Each same was taken to the lab, and itschloride content was determined as a function of time elapsed fromsampling. The results are presented in Table II. The Ex. 6, data showessentially constant levels of chloride ion, indicating that labile,chloride-generating species such as methyl chloroformate are not presentin the sample. In contrast, the data for Comp. Ex. 1 show an increasewith chloride level over time, consistent with presence of methylchloroformate in the initial sample and its decomposition over time toform additional chloride ion. Thus, the data collectively show that inthe absence of the fluid passageway 110, substantial chloride formationmay take place in downstream (post-acid removal column 160) sections ofthe plant, causing corrosion, but the presence of the fluid passageway110 is effective to decompose alkyl chloroformate to chloride ion beforethe acid removal column 160, thereby preventing downstream corrosion.[t5] TABLE II Chloride Concentration (ppm) Time (h) Ex. 6 Comp. Ex. 1 0374 189 2 408 312 4 374 339 8 372 368 10 372 357 25 381 368

EXAMPLE 7, COMPARATIVE EXAMPLE 2

[0071] For Comparative Example 2, a dimethyl carbonate plant accordingto simplified FIG. 2 was operated according to the conditions describedin Table III, below. This plant was similar to that shown in more detailin FIG. 1, with the exception that the first heat exchanger 140 and thefluid passageway 110 were absent. Corrosion was observed in anddownstream of the azeotrope column 180. Next, this plant was modified toinclude the first heat exchanger 140 and two holding vessels 120 wereadded to increase residence time (i.e., FIG. 3 configuration). FIG. 10presents measurements of residual ionic chlorides found in samples takenfrom the bottom of the azeotrope column 180, comparing the FIG. 2 andFIG. 3 configurations, each over time. Residual chlorides weredetermined by titration using a silver nitrate solution, as describedabove. The data for the FIG. 2 configuration have an average of 671 ppmchloride with a standard deviation of 370 ppm chloride, whereas the datafor the FIG. 3 configuration have an average of 35 ppm chloride and astandard deviation of 25 ppm chloride. The data thus show a dramaticreduction in chloride levels for the FIG. 3 configuration vs. the FIG. 2configuration. It is predicted this reduction would be even greater forthe configurations according to FIGS. 4 and 6, in which four holdingvessels 120 are used to provide a residence time of four hours at 70° C.FIG. 11 presents measurements of methyl chloroformate concentrationentering and exiting the fluid passageway 110 of the FIG. 3concentration. In other words, the points signified by “+” and labeled“MCF feed to Phase 0” in FIG. 11 correspond to measurements on themixture as it was entering the fluid passageway 110; these points havean average value of 930 parts per million by weight (ppmw) and astandard deviation of 412 ppmw. And the points signified by “▪” andlabeled “MCF from Phase 0” correspond to measurements on the mixture asit exits the fluid passageway 110; these points have an average value of45 ppmw and a standard deviation of 77 ppmw. These data clearly showthat an apparatus according to FIG. 3 is effective to dramaticallyreduce the concentration of methyl chloroformate in the process stream.

[0072] [t1] TABLE III Ex. 7 (FIG. 2 Control Ex. 2 (FIG. 3 ConditionsConfiguration) Configuration) Mass Ratio MeOH/O₂/CO 0.7/0.06/10.7/0.06/1 Catalyst Content Fixed Fixed Reaction Temperature (° C.) 133 133 Reaction Pressure (barg) 23 23 Temp. of Pre-Residence Time Heater 60— (° C.) Temp. of Acid Column Feed Vaporizer 90 90 (° C.) Residence Timebetween flash vessel  2 0.03 and acid column, excluding both (hours)

[0073] [t2] TABLE IV average chloride concentration ± standardConfiguration deviation (ppm) FIG. 3 (comparison) 671 ± 370 FIG. 2(invention) 35 ± 25

[0074] While the invention has been described with reference to apreferred embodiment, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing fromessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

[0075] Where not specifically defined herein, technical terms in thisspecification may be interpreted according to Grant and Hach's ChemicalDictionary, 5^(th) ed., McGraw-Hill, Inc.

[0076] All cited patents and other references are incorporated herein byreference in their entirety.

1. A method of preparing dimethyl carbonate, comprising: reactingmethanol, oxygen, carbon monoxide, and a catalyst to form a mixturecomprising dimethyl carbonate, methyl chloroformate, hydrochloric acid,water, carbon dioxide, and carbon monoxide; and subjecting said mixtureto conditions comprising a water concentration ([H₂O]), a methanolconcentration ([MeOH]), a temperature (T), and a residence time (t),such that a parameter X calculated according to the expressionX=exp{−[(2.09×10⁹)e ^((−6381/T))[H₂O]+(4.14×10¹⁰)e ^((−7673/T))[MeOH]]t} has a value less than about 0.9, wherein said water concentration andsaid methanol concentration are expressed in moles per milliliter, saidtemperature is expressed in degrees Kelvin, and said residence time isexpressed in minutes.
 2. The method of claim 1, wherein said parameter Xhas a value less than about 0.2.
 3. The method of claim 1, wherein saidparameter X has a value less than about 0.1.
 4. The method of claim 1,wherein said parameter X has a value less than about 0.05.
 5. The methodof claim 1, wherein said parameter X has a value less than about 0.01.6. The method of claim 1, wherein said water concentration is about 0.1to about 50 moles per liter.
 7. The method of claim 1, wherein saidmethanol concentration is about 1 to about 25 moles per liter.
 8. Themethod of claim 1, wherein said temperature is about 30° C. to about130° C.
 9. The method of claim 1, wherein said residence time is about0.5 hour to about 10 hours.
 10. The method of claim 1, furthercomprising removing hydrochloric acid from said mixture.
 11. The methodof claim 10, wherein said removing hydrochloric acid comprises reducingthe concentration of said hydrochloric acid to less than about 1×10⁻³moles per liter.
 12. A method of preparing dimethyl carbonate,comprising: reacting methanol, oxygen, carbon monoxide, and a catalystto form a mixture comprising dimethyl carbonate, methyl chloroformate,hydrochloric acid, water, carbon dioxide, and carbon monoxide;subjecting said mixture to conditions comprising an initialconcentration of methyl chloroformate ([MCF]_(t=0)), a waterconcentration ([H₂O]), a methanol concentration ([MeOH]), a temperature(T), and a residence time (t), such that a parameter Z calculatedaccording to the expression Z=[MCF]_(t=0) exp{−[(2.09×10⁹)e^((−6381/T))[H₂O]+(4.14×10¹⁰)e ^((−7673/T))[MeOH]]t}  has a value lessthan about 5×10⁻⁶, wherein said initial concentration of methylchloroformate, said water concentration, and said methanol concentrationare expressed in moles per milliliter, said temperature is expressed indegrees Kelvin, and said residence time is expressed in minutes.
 13. Themethod of claim 12, wherein said initial concentration of methylchloroformate is about 5×10⁻⁶ to about 5×10⁻⁴ moles per liter.
 14. Themethod of claim 12, further comprising removing hydrochloric acid fromsaid mixture.
 15. A method of preparing a dialkyl carbonate, comprising:reacting methanol, oxygen, carbon monoxide, and a catalyst to form amixture comprising dimethyl carbonate, methyl chloroformate,hydrochloric acid, water, carbon dioxide, and carbon monoxide; andmaintaining said mixture at a temperature of about 50° C. to about 80°C. for about 1 hour to about 10 hours.
 16. The method of claim 15,further comprising removing hydrochloric acid from said mixture.
 17. Amethod of preparing dimethyl carbonate, comprising: reacting methanol,oxygen, carbon monoxide, and a catalyst to form a mixture comprisingdimethyl carbonate, methyl chloroformate, hydrochloric acid, water,carbon dioxide, and carbon monoxide; and subjecting said mixture toconditions comprising an initial dimethyl carbonate concentration([DMC]_(t=0)), an initial water concentration ([H₂O]_(t=0)), an initialmethanol concentration ([MeOH]_(t=0)), an initial hydrochloric acidconcentration ([HCl]_(t=0)), a temperature (T), and a residence time(t), such that a parameter X calculated according to the expressionX=exp{−[(2.09×10⁹)e ^((−6381/T)[H₂O]_(t=0)(4.14×10¹⁰)e^((−7673/T))[MeOH]_(t=0) ]t}  has a value less than about 0.9, and aparameter Y calculated according to the expression$Y = \frac{( {1 - \frac{\lbrack {H_{2}O} \rbrack_{t = 0}}{\lbrack{DMC}\rbrack_{t = 0}}} )}{\begin{matrix}( {1 - {( \frac{\lbrack {H_{2}O} \rbrack_{t = 0}}{\lbrack{DMC}\rbrack_{t = 0}} )( {\exp( ( {6.6 \times 10^{10}} ) } }}  \\   {{{( {\exp ( {{- 6636}/T} )} )\lbrack{HCl}\rbrack}_{t = 0}\lbrack{DMC}\rbrack}_{t = 0}( {\frac{\lbrack {H_{2}O} \rbrack_{t = 0}}{\lbrack{DMC}\rbrack_{t = 0}} - 1} )t} ) ) )\end{matrix}}$

 has a value of at least about 0.9, wherein said initial dimethylcarbonate concentration, said initial water concentration, said initialmethanol concentration, and said initial hydrochloric acid concentrationare expressed in moles per milliliter, said temperature is expressed indegrees Kelvin, and said residence time is expressed in minutes.
 18. Themethod of claim 17, wherein said parameter Y has a value of at leastabout 0.95.
 19. The method of claim 17, wherein said parameter Y has avalue of at least about 0.99.
 20. The method of claim 17, wherein saidinitial dimethyl carbonate concentration is about 0.5 to about 10 molesper liter.
 21. The method of claim 17, wherein said initial hydrochloricacid concentration is about 1×10⁻³ to about 2×10⁻¹ moles per liter. 22.The method of claim 17, further comprising removing hydrochloric acidfrom said mixture.
 23. A method of preparing dimethyl carbonate,comprising: reacting methanol, oxygen, carbon monoxide, and a catalystto form a mixture comprising dimethyl carbonate, methyl chloroformate,hydrochloric acid, water, carbon dioxide, and carbon monoxide; andsubjecting said mixture to conditions comprising a water concentration([H₂O]), a methanol concentration ([MeOH]), a temperature (T), and aresidence time (t), such that a parameter X′ calculated according to theexpression X′=1/└1+[(2.09×10⁹)e ^((−6381/T))[H₂O]+(4.14×10¹⁰)e^((−7673/T))[MeOH]]t┘ has a value less than about 0.5, wherein saidwater concentration and said methanol concentration are expressed inmoles per milliliter, said temperature is expressed in degrees Kelvin,and said residence time is expressed in minutes.